In the direct reduction of iron oxide to metallized iron in a shaft furnace, particulate iron oxide is fed to the top of the furnace and particulate metallized product is removed from the bottom, which creates a gravitational flow of the burden through the furnace. Hot reducing gas is introduced to the burden intermediate the ends of the furnace and in most shaft furnace processes, a cooling gas is introduced to the burden near the discharge end of the furnace to cool the metallized product prior to discharge, after which the heated cooling gas is removed from the cooling zone, cleaned, cooled, and recirculated.
During the reduction process, stresses are created within the particles of material being reduced, primarily due to a change in crystal structure during reduction from hematite to magnetite. Particulates in the form of pellets or natural lump ores have adequate elasticity to accommodate these stresses; however, some finely divided particles of material are created. These are known as fines.
About one percent of the total output of a direct reduction shaft furnace is removed as solid fines entrained in the cooling zone gas withdrawn from the furnace. The entrained solids are separated from the gas in a cooling gas cooler-scrubber as sludge, or they are carried into a clarifier and discharged as sludge with the clarifier underflow. Such sludge has a high percentage of iron; therefore, it is desirable to reclaim the iron by agglomerating and reprocessing the material in the direct reduction furnace. Alternatively, the sludge may be utilized as feed for a sintering machine. Recently such sludge has been briquetted with dry dust created elsewhere in the direct reduction process. This latter method is a lower-cost recovery method where inexpensive ores which generage large amounts of fines are being used.
Where direct reduction plants operate with higher quality ores which produce lower quantities of metallized fines, these fines may be injected in limited amounts directly into electric arc steelmaking furnaces. When fines are not briquetted it is desirable to collect the maximum amount of dry iron fines possible. By maintaining the fines in the dry state, the agglomeration of fines due to rusting (rust bonding) is avoided.